A small world

Image from a cryo-electron tomography 3D reconstruction revealing structure morphological features of an ultra-small bacterium. The high contrast sub-cellular bodies located at cell ends are putative ribosomes. Credit@LuisComolli

Researchers from the U.S. at the Lawrence Berkeley National Laboratory and the University of California captured the first high-resolution microscopy images of the smallest living organism on the planet. It is believed to be as small as life may physically get. The existence of these ‘ultra-small’ organisms has been speculated for decades. A paper published in the journal Nature Communications confirms the suspicion through detailed microscopy techniques.

The tiny bacteria discovered by the scientists have a cell size of just 0.009 micrometres cubed (or 0.000009 millimetres cubed). At that size, it would take approximately 150 of these bacteria to fill an E. coli cell, and 150,000 could squeeze onto the end of a single hair.

The team collected the bacteria from groundwater at Rifle, Colorado, leading researchers to suggest that they may be relatively common. By using successively smaller filters (small enough to sterilise water) they found samples enriched with unbelievably small microbes. They are thought to be at the minimum size limit for life – the smallest a microorganismal cell may be and still have enough space for the essential materials necessary to sustain life. Once captured, they were cryogenically frozen in a ‘cryo plunger’ to protect them on the way to Berkeley lab. The frozen samples were examined under 2-D and 3-D cryogenic transmission electron microscopy.

Scientists observed a number of typical bacterial features including DNA, a small number of ribosomes (which assemble proteins inside the cell) and tiny “hairs” on the surface known as pili. Most distinguishingly though, the paper describes the cell as having a stripped-down metabolism that may require the cells to depend upon other microorganisms for many important biological processes, possibly by using the pili. Dr. Luis Comolli of Berkeley Lab’s Life Sciences Division explains that, “The most significant finding, however, is that there seem to be numerous microorganisms without complete metabolic machinery, which are unattached to other, more complex, microbes. Remarkably, they must share metabolic intermediates and other biological macromolecules within a vast web of life.”

This lab-based work, combined with cutting-edge genetic analysis, produced the most comprehensive account of these creatures to date. The genome was sequenced and found to be approximately one million base pairs in length. Further genomic analysis revealed that the bacteria collected from the ground water actually comprised of three separate phyla: WWE3, OP11 and OD1.

This research is a milestone contribution to the field of ultra-small organisms and sheds new light on the “nanobacteria” debate. Nanobacteria are a proposed class of living organisms with a size much smaller than expected by the minimum size limit for life at around 200 nanometres for bacteria (or 0.2 micrometres). Professor Jill Banfield, a Senior Faculty Scientist in Berkeley Lab’s Earth Sciences Division, told The Positive, “An angle that I think has been largely missed in the press coverage is the direct relevance of this work to the nanobacteria controversy.” Experts have often appeared to dismiss the notion of these living microbes, however the advanced microscopy techniques demonstrated in this study present proof that nanobacteria might exist after all.

Scientists believe that these ultra-small bacteria might play important roles in the ecosystems that they inhabit. However, since little is currently known about them scientists are trying to determine what they even do. Banfield goes on to say, “They’re enigmatic. These bacteria are detected in many environments and they probably play important roles in microbial communities and ecosystems.” The ultra-small organisms are subset of life nobody knows very much of, even though they might have existed for a very long time; there is a whole new world of life to explore.

How might these ‘ultra-small’ organisms improve the ecosystems they inhabit, including their interaction with human life?

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